The present invention relates to the field of digital communications, and more specifically, is directed to a system for acquiring or recovering a symbol timing offset and carrier frequency error from a digitally modulated, multi-carrier communications signal. The system of the present invention is further directed to a diversity system for recovering a symbol timing offset and carrier frequency error from a diverse, multi-frequency digitally modulated signal.
In a digital communication system, the acquisition of symbol synchronization between the transmitter and receiver must be achieved before effective symbol demodulation can commence at the receiver. Thus, the prior art in digital communications is replete with attempts to develop systems directed to the optimal recovery or acquisition of received symbol timing and carrier frequency.
One such attempt is provided in U.S. Pat. No. 5,541,552, by Suzuki, directed to the demodulation of digitally modulated multi-carrier symbols. The receiver in that patent develops correlation peaks responsive to an intra-symbol correlation, as by conjugate product formation, between the repetitive information and the amplitude tapering existing at the leading and lagging portions of each received symbol. The peaks must be detected, for their positions are indicative of received symbol timing. However, their relatively flat amplitude profiles necessitate their individual integration prior to the peak detection thereof.
This simplistic signal processing scheme in Suzuki, i.e., mere integration of correlation peaks prior to their detection, provides sub-optimal synchronization results on at least two accounts. First, as the number of frequency carriers is increased in the multi-carrier symbol, the correlation peak becomes obscured by adjacent noise, and mere integration fails to sufficiently enhance the signal-to-noise ratio of the pre-detected correlation peaks. Second, mere integration fails to combat well known signal propagation effects, such as scattering, fading, or other signal interferers, all of which combine to cause signal drop-outs and false peaks.
In contrast to Suzuki and other prior art acquisition systems, the system of the present invention presents an optimum scheme for the recovery of symbol timing and carrier frequency. This optimum approach is provided through an application of signal processing techniques that, when taken either alone or in combination, improve over the prior art. These signal processing improvements include the additive superposition of repeating signals, optimum or matched filtering, and diversity selection and combining between diverse, modulated signals.
The following is a list of prior art references known to Applicant: U.S. Pat. Nos. 5,694,389; 5,602,835; 5,608,764; 5,559,833; 5,687,165; 5,541,552; 5,357,502; 3,925,650; 5,596,582; 3,364,482; 2,943,316; 3,975,687; 5,594,761; 4,281,412; 5,187,711; 4,727,534; 5,369,800; 5,548,819; 2,549,423; 2,880,275; 3,555,427; 5,629,639; 5,428,647; 5,682,376; 5,416,767; 5,452,331; 4,344,180; 5,657,313; 5,652,772; 5,627,863; 5,550,812; 5,506,836; 5,471,464; 5,450,456; 5,371,761; 5,345,440; 5,313,169; 5,228,025; 5,191,576; 5,371,548; 5,406,551; and, 3,780,279.